As the bacterium Bacillus subtilis differentiates from the vegetative form into a dormant endospore, complex morphological and physiological changes occur which require the sequential expression of many genes. During the process, new RNA polymerase sigma subunits appears, displacing one another in a sequential cascade and conferring on the RNA polymerase a changing specificity for the recognition of different classes of promoters. This mechanism of altering transcriptional specificity may be a fundamental element in the regulation of sporulation gene expression. Experiments are proposed to determine how sigma factors work to change the specificity for the recognition of promoters by RNA polymerase and to determine how expression of a sporulation-induced sigma is regulated. Additional experiments are proposed to characterize a recently discovered non-sigma regulatory factor and to determine the signals that control the compartmentalization of gene expression. These studies are directly relevant to understanding the regulation of differentiation and secondary metabolism in a broad range of bacteria. Furthermore, the biochemical principles that enable a protein to direct RNA polymerase to specific sites on DNA are probably relevant to RNA polymerases in all organisms.